Stack-type diode laser device

ABSTRACT

Two thin-clad laser diodes are disposed to form a stack-type diode laser device. The diodes emit two beams that are substantially parallel and in proximity such that they share many fiberoptic systems designed for a single beam. The diodes are coupled by leaky waves through top surfaces. The leaky waves are generated by a thin metal contact layer or diffractive gratings. The stack-type device is employed for single-mode power enhancement and tunable lasers.

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application is related to my U.S. regular patent applicationSer. No. 10/285,022, filed on Oct. 30, 2002, which is incorporatedherein by reference.

[0002] This application claims the benefit under 35 U.S.C. sec. 119 ofprovisional application Ser. No. 60/469,306, filed May 9, 2003.

FEDERALLY SPONSORED RESEARCH

[0003] Not applicable

SEQUENCE LISTING OR PROGRAM

[0004] Not applicable

BACKGROUND

[0005] 1. Field of Invention

[0006] This invention relates to semiconductor lasers, and particularlyto stack-type semiconductor laser devices.

[0007] 2. Description of Prior Art

[0008] As used here, the term “semiconductor laser” or “diode laser”means edge-emitting diode lasers. Edge-emitting diode lasers have ahorizontal cavity and emit light beams in a direction parallel to thewafer's plane or an active region of the wafer. Most semiconductorlasers in use are edge-emitting and are used for laser printers,fiberoptic telecommunication systems, and optical data storage devices.

[0009] A diode emits an output light beam with a spatial powerdistribution. The output of a single transverse mode diode, or asingle-mode diode, has a single lobe in its spatial power distribution,while a multimode output has multiple lobes. Single-mode diode lasersare desirable in many fields, especially in fiberoptictelecommunications and optical data storage. However one of theweaknesses of single-mode diode lasers is that they have relatively lowoutput power.

[0010] The importance of tunable wavelength diode lasers has growndramatically in fiberoptic telecommunications in recent years. Tunablelasers have three important specifications, i.e., output power,wavelength tuning range, and tuning speed. A typical high output poweris about 20 milliwatts. A typical wide tuning range is about 40nanometers, e.g., the diode can emit light from 1525 to 1565 nm. A fasttuning speed is in a range of several nanoseconds. Among various tunablelasers, distributed feedback (DFB) lasers offer high power, but suffer anarrow tuning range and a slow tuning speed. Three-section distributedBragg reflector (DBR) lasers have a fast tuning speed and moderatepower, but a narrow tuning range. Sampled grating DBR (SG-DBR) lasersand superstructure grating DBR (SSG-DBR) lasers have a wide tuning rangeand a fast tuning speed, but low output power. Conventional tunableexternal cavity diode lasers benefit from high power and a wide tuningrange, but suffer a slow tuning speed. Thus current tunable lasers can'tsatisfy the three specifications: high power, wide tuning range, andfast tuning speed.

[0011] One approach to overcome limitations on the single-mode outputpower and tunable laser involves stacking two thin-clad laser diodes toform a stack-type diode laser device and coupling the diodes, as isdisclosed in my above regular U.S. patent application. The thin-claddiodes emit two beams which are substantially parallel and proximatesuch that they can simultaneously feed a fiberoptic system designed fora single beam. When the diodes are coupled in phase, their outputs canbe combined to increase single-mode power, or novel tunable lasers canbe created to meet the three specifications. However, the disclosedcoupling mechanisms rely on external feedbacks, which require additionaloptics. The resulting stack-type device is complicated and bulky.

OBJECTS AND ADVANTAGES

[0012] Accordingly, several main objects and advantages of the presentinvention are:

[0013] (a). to provide an improved stack-type diode laser device;

[0014] (b). to provide such a device which has a simple and compactstructure;

[0015] (c). to provide such a device which has a simple and compactstructure and generates higher output power than that obtainable from asingle laser diode;

[0016] (d). to provide a tunable laser which has a simple and compactstructure and has high power, a wide tuning range, and a fast tuningspeed.

[0017] Further objects and advantages will become apparent from aconsideration of the drawings and ensuing description.

SUMMARY

[0018] In accordance with the present invention, two thin-clad diodesare stacked together with the top surfaces opposite each other. Thediodes are coupled by leaky waves through their top surfaces. The leakywaves can be generated by a thin metal contact layer or diffractivegratings. The resulting device is simple, compact, and can be used tocombine output power of the diodes or create a tunable laser which hashigh power, a wide tuning range, and a fast tuning speed.

[0019] Abbreviations

[0020] DBR Distributed Bragg reflector

[0021] DFB Distributed feedback

[0022] SG-DBR Sampled grating-distributed Bragg reflector

[0023] SSG-DRR Superstructure grating-distributed Bragg reflector

DRAWING FIGURES

[0024]FIG. 1-A is a schematic cross-sectional view of a typicalprior-art ridge-guide laser diode.

[0025]FIG. 1-B is a schematic perspective view of a typical prior-artridge-guide laser diode.

[0026]FIG. 1-C is a schematic cross-sectional view of a prior-artstack-type diode laser device.

[0027]FIG. 2-A is a schematic cross-sectional view illustrating anembodiment of a stack-type diode laser device according to theinvention.

[0028]FIG. 2-B is a schematic cross-sectional view illustrating astack-type diode laser device coupled to a single-mode fiber accordingto the invention.

[0029]FIGS. 3 and 4 are schematic cross-sectional views illustratingembodiments of a thin-clad laser diode having different diffractiongratings on the top surface according to the invention.

Reference Numerals in Drawings

[0030]10 active region

[0031]12 p-cladding layer

[0032]14 n-cladding layer

[0033]15 insulator region

[0034]16 contact layer

[0035]17 substrate and cladding layer

[0036]18 metal contact layer

[0037]19 metal contact layer

[0038]20 n-type substrate

[0039]21 substrate and cladding layer

[0040]22 light-emitting spot

[0041]24 active region

[0042]25 active region

[0043]26 cladding and contact layer

[0044]28 cladding and contact layer

[0045]30 grating element

[0046]32 grating element

[0047]34 grating element

[0048]36 grating element

[0049]37 thin-clad diode

[0050]38 thin-clad diode

[0051]40 thin-clad diode

[0052]42 thin-clad diode

[0053]44 output beam

[0054]46 output beam

[0055]48 lens system

[0056]50 single-mode fiber

[0057]52 anti-reflection coating

[0058]54 single-mode fiber

DETAILED DESCRIPTION

[0059] FIGS. 1-A TO 1-C—Prior-Art

[0060] FIGS. 1-A and 1-B show schematic cross-sectional and perspectiveviews of a typical prior-art ridge-guide laser diode. The diode isfabricated on an n-type substrate 20, which usually is a thin square orrectangular semiconductor chip with a thickness around one hundredmicrons. Deposited on substrate 20 are an n-type cladding layer 14, anactive region 10 of the diode, where light is generated, a p-typecladding layer 12, a p-type contact layer 16, two separate insulatorregions 15, and a metal contact layer 18 as a top electrode. The diodeemits light beams from an elliptical region 22 on the edge or side ofthe diode.

[0061] As shown in the figures, a protrusion, ridge, or boss is formedby etching layer 16 and part of layer 12 in the center portion of thechip. A top surface comprises the upward surface area of the protrudedpart of layer 18. The layers between active region 10 and the topsurface have a total layer thickness of about two microns for a regularthick-clad diode. In the case of a thin-clad design, the total layerthickness can be reduced to several tenths of one micron.

[0062]FIG. 1-C illustrates schematically a prior-art stack-type diodelaser device. The arrangement comprises a lens system 48, a single-modefiber 50, and thin-clad diodes 40 and 42 which generate respectiveoutput beams 44 and 46. The beams are coupled into fiber 50 by lenssystem 48. The diodes are opposite each other, or in other words, theirtop surfaces face each other. Due to the nature of thin-clad diode, whendiodes 44 and 46 are disposed such that they are in close distance,beams 44 and 46 can be arranged to be substantially parallel andproximate. For example, the separation between the two beams can besmaller than one micron. In such a case, two beams at 1.55-micronwavelength can share lens system 48 and fiber 50 simultaneously with arelatively small additional coupling loss, which is about 0.2 dBcomparing to a case where a single diode is coupled to a single-modefiber through a coupling lens.

[0063] However, the diodes in FIG. 1-C don't interact with each otherdirectly. In the prior-art, the diodes are coupled by feedbacks whichcoupling optics generates. The coupling optics makes the stack-typedevice complicated and bulky.

[0064]FIG. 2-A—Stack-Type Device with a Thin Metal Contact Layer

[0065]FIG. 2-A shows schematically a cross-sectional view of a preferredembodiment of a stack-type device according to the invention (Supportingand bonding structures are not shown). The cross section is cut along adirection of light propagation. Thin-clad ridge-guide diodes 37 and 38are opposite and each have a similar layered structure to that of FIG.1-A. Layers 18 and 19 are thin metal contact layers. Active regions 24and 25 lie between regions 26 and 28 and regions 17 and 21,respectively. Regions 26 and 28 contain cladding and contact layers.Regions 17 and 21 contain cladding and substrate layers. The diodes aredisposed such that their top surfaces are proximate and their outputbeams (not shown) are parallel to a certain degree.

[0066] Because of the thin-clad design, light waves propagating insidethe diode interact with the metal contact layer in a greater degree thana thick-clad diode. Parts of the light waves are reflected back by themetal layer, and parts of them leak out through it. Since the diodes'top surfaces are opposite each other, a portion of the leaking waves ofone diode enters the other diode and vice versa. Thus each diode has aportion of its light wave coupled in the other diode. When the couplingintensity is strong enough, the diodes influence each other in terms ofphase and mode selection of their propagating light waves. The lightwaves inside the diodes can be locked in phase.

[0067] The coupling efficiency of the two diodes depends upon theintensity of the leaking waves. The leakage intensity in turn dependsupon property of the metal contact layer, the metal layer thickness, andthe diode structure. The thinner the metal layer, the larger theleakage. But the metal contact layer can't be too thin. First, leakagecauses power loss; second, the metal contact layer is responsible forelectrical contact so it must have an adequate thickness.

[0068]FIG. 2-B—Stack-Type Device Coupled to a Single-Mode Fiber

[0069] When two diodes of a stack-type device are coupled in phasedirectly, the device can have a simple and compact structure to providepower enhancement and a wavelength tuning mechanism. FIG. 2-B showsschematically a cross-sectional view of an embodiment of a stack-typedevice according to the invention. Thin-clad diodes 37 and 38 are phaselocked by leaking waves through their top surfaces. Their output beamsare coupled into a single-mode fiber 54 by lens system 48. Fiber 54 hasan anti-reflection coating 52 on its angled end to reduce unwantedfeedback, which can cause instability of output power and wavelength.

[0070] When light waves in diodes 37 and 38 are in phase, constructiveinterference occurs between their output beams and their output power iscombined. The diodes can be of same or different types that have athin-clad design and a thin metal contact layer, e.g., Fabry-Perot, DFB,DBR, or a semiconductor amplifier, which is sometimes called a gainchip, as along as their output spectra partially overlap. When a DBFdiode having a narrow spectral width around 1550 nm and a broadbanddiode having a spectral range from 1535 to 1565 nm are coupled, the DFBdiode dominates output wavelength of the broadband diode, and thediodes' power are combined at the DFB's wavelength. The stack-typedevice in FIG. 2-B is simple and compact and achieves larger single-modeoutput power than a single diode.

[0071] It is well known to those skilled in the art that phase lockingbetween two diodes in such stack-type device is affected by diodecharacteristics and the spacing between the two top surfaces besidescoupling efficiency. Therefore diode structure and dimensions and thespacing must be optimized. Moreover, drive current and temperature ofthe diodes can be used to fine tune phase relation between the diodes.In a passive phase locking mechanism, values of the drive current andtemperature are chosen and fixed. In an active case, the values areadjusted in a feedback loop according to output power received by apower monitor.

[0072] The embodiment of FIG. 2-B can also be used to create schemes totune the output wavelength. The tuning mechanism is quite similar tothat of a SG-DBR or a SSG-DBR laser. SG-DBR and SSG-DBR lasers have aphase section, which fine tunes the phase, a gain section, whichamplifies the light, and two Bragg reflector sections, which reflectlight and feature a comb-like reflective spectrum. The output wavelengthis selected by matching one peak of one comb to another peak of theother comb. To tune the wavelength, at least one spectrum is moved sothat the two peaks coincide at another wavelength. Since a Braggreflector causes considerable power loss and is difficult to fabricate,SG-DBR and SSG-DBR lasers, each having two of such reflector, sufferfrom low power and yield concerns.

[0073] To employ the embodiment of FIG. 2-B as a tunable laser similarto SG-DBR and SSG-DBR lasers, diodes 37 and 38 are designed to have aphase section, a gain section, and a Bragg reflector section which islocated close to the diode's rear facet. The Bragg reflector gives thediodes a comb-like output spectrum. Since the diodes are coupled, theiroutput wavelength can be generated by matching two peaks of thecomb-like spectra. The resulting tunable laser has a similar fast tuningspeed and a similar wide tuning range to SG-DBR and SSG-DBR lasers. Butsince each diode of the stack-type device has only one Bragg reflector,it has larger output power and a better yield. In addition, the totaloutput power is further increased by constructive interference of thetwo diodes. Therefore the stack-type diode laser device produces highoutput power, fast tuning speed, and wide tuning range in a simple andcompact structure.

[0074]FIGS. 3 and 4—Thin-Clad Diodes with Diffraction Gratings

[0075] Besides a thin metal contact layer, there are other ways forlight waves to leak out through the top surface of a stack-type diodedevice. FIG. 3 shows schematically a cross-sectional view of a thin-claddiode having diffraction gratings on its top surface according to theinvention. Like diode 38 in FIG. 2-A, the diode has region 28 containingcladding and contact layers, active region 25, and region 21 containingcladding and substrate layers. As in FIG. 3, an x-axis lies in the lightpropagation direction and a z-axis in a direction perpendicular to thetop surface. A grating area usually consists of repetitive arrays ofgrating elements. In FIG. 3, grating elements 30 and 32 are arrangedalong x-axis. Grating element 30 represents a metal row, which is madeby selectively etching or depositing metal contact layer. Element 32represents a row without the metal layer. The rows are parallel to they-axis (Not shown). A grating period L is the width addition of the twotypes of rows. All the metal rows are connected electrically.

[0076] When interacting with an impinging light wave, diffractivegratings generate multiple waves in different directions. Each directioncorresponds to a diffraction order which is represented by an integer,e.g., 0, 1, −1, 2, −2, etc. Assume that the diode of FIG. 3 emits a beamhaving a wavelength λ and has an effective refractive index n for λ.According to grating theories well known in the art, when the gratingsin FIG. 3 satisfy 2λ=2nL, which is the second order, they also satisfy afirst-order diffraction by the relation of λ=nL. By the second-orderdiffraction, a portion of the light is diffracted along x-axis.Meanwhile, the first-order diffraction sends another portion of thelight out through the top surface along z-axis. Although thesecond-order gratings function as distributed Bragg reflector (DBR), thegratings are often called a grating coupler since light waves travelinginside the diode can be coupled out vertically.

[0077] When two DFB or DBR diodes in a stack-type structure employ thesecond-order gratings, they can be coupled by the first-orderdiffraction when their Bragg reflector sections are aligned. When thediodes are the same, the spacing between their top surfaces must beoptimized so that the light waves in the diodes are in phase. Whendifferent diodes are used, their structure and dimensions must beoptimized, too.

[0078]FIG. 4 shows schematically a cross-sectional view of anotherthin-clad diode having diffraction gratings on its top surface accordingto the invention. The cross section is cut along a direction of lightpropagation in the diode. The diode of FIG. 4 is similar to the diode ofFIG. 3 but has a different grating structure. In FIG. 4, a gratingelement 36 represents a row without a metal layer, and a grating element34 a metal row; the latter is made by etching part of the contact layer,then depositing a metal contact layer on the same place. As in FIG. 3,all the metal rows are connected electrically. Since element 34 iscloser to the active region than element 32 in FIG. 3 for the samediode, it encounters stronger light waves propagating in the diode. As aresult, the first-order diffraction and the leaky waves become strongerthan in FIG. 3.

[0079] Moreover, light waves in a thin-clad diode can leak out throughan area on its top surface where the metal contact layer is etched. Twosuch diodes in a stack-type structure can be coupled as in the abovecases of thin metal contact layer or diffraction gratings.

CONCLUSION, RAMIFICATIONS, AND SCOPE

[0080] Accordingly, the reader will see that two thin-clad diodes of astack-type device can be coupled by leaky waves through their topsurfaces. The resulting stack-type diode laser device has a simple andcompact structure. It produces a larger single-mode output than a singlediode. In addition, it provides a tunable laser that has a wide tuningrange, a fast tuning speed, and high output power.

[0081] Although the above description contains many specificities, theseshould not be construed as limiting the scope of the invention, but asmerely providing illustrations of some of the presently preferredembodiments. Numerous modifications, alternations, and variations willbe obvious to those skilled in the art.

[0082] For example, the diodes of a stack-type device can have differentleaking structures, as long as adequate coupling efficiency is achieved.

[0083] Furthermore, the diffractive gratings in FIGS. 3 and 4 can bedisposed beneath the top surface besides on it. When the gratings arebelow the top surface, light waves propagating inside a diode can leakor be coupled out through its top surface for both thin-clad andthick-clad diodes. Then two diodes having a thick clad can be coupled ina stack-type structure, too. Although the coupled thick-clad diodesdon't share an optical system designed for a single beam with a lowpower loss as two thin-clad diodes in a stack-type device, they providetwo beams in a stable phase relation, which has applications in opticalcoherent measurements.

[0084] Therefore the scope of the invention should be determined by theappended claims and their legal equivalents, rather than by the examplesgiven.

1. A semiconductor laser device comprising: 1) a first semiconductorlaser diode arranged to emit an output beam having a predeterminedoutput spectrum for use at a predetermined wavelength, said diodeincluding: a) atop surface, b) a light generating substructure includinga plurality of semiconductor layers disposed below said top surface, b)said light generating substructure having an active region and alight-emitting spot, c) a leakage substructure arranged such that apredetermined portion of said generated light leaks out through said topsurface; 2) a second semiconductor laser diode arranged to emit anoutput beam having a predetermined output spectrum for use at apredetermined wavelength, said diode including: a) a top surface, b) alight generating substructure including a plurality of semiconductorlayers disposed below said top surface, b) said light generatingsubstructure having an active region and a light-emitting spot, c) aleakage substructure arranged such that a predetermined portion of saidgenerated light leaks out through said top surface; and 3) bonding andsupporting means for disposing said first and second laser diodes suchthat their top surfaces are facing each other.
 2. The laser deviceaccording to claim 1 wherein said first and second laser diodes aredisposed such that their output beams have substantially parallelirradiation directions.
 3. The laser device according to claim 1 whereinsaid first and second laser diodes are arranged such that apredetermined portion of said leaking light of each of said first andsecond laser diodes enters said light generating substructure oppositeto said leaking light.
 4. The laser device according to claim 1 whereinsaid first and second laser diodes are constructed so that said activeregion and said light-emitting spot are proximate to said top surface.5. The laser device according to claim 1 wherein said first and secondlaser diodes are disposed such that their top surfaces are proximate. 6.The laser device according to claim 1 wherein at least one of saidleakage substructures of said first and second laser diodes comprises aplurality of diffractive gratings, said diffractive gratings arearranged such that said predetermined portion of said generated lightleaks out through said top surface.
 7. The laser device according toclaim 1 wherein at least one of said leakage substructures of said firstand second laser diodes comprises a relatively thin metal contact layer,said metal contact layer is arranged such that said predeterminedportion of said generated light leaks out through said top surface. 8.The laser device according to claim 1, further including tuning meansfor tuning the output wavelength of said first and second laser diodeswithin a predetermined wavelength range, said tuning means beingarranged to tune at least one of said laser diodes.
 9. The laser deviceaccording to claim 1, further including coupling means for coupling saidfirst and second laser diodes, said coupling means comprising phaselocking means for locking said generated light of said first and secondlaser diodes in a predetermined phase relation.
 10. The laser deviceaccording to claim 1, further including lens means for coupling saidoutput beams of said first and second laser diodes into a single-modedevice.
 11. A method for coupling two semiconductor laser diodes, saidmethod comprising: A. providing a semiconductor diode device, said diodedevice comprising: 1) a first semiconductor laser diode arranged to emitan output beam having a predetermined output spectrum for use at apredetermined wavelength, said diode including: a) a top surface, b) alight generating substructure including a plurality of semiconductorlayers disposed below said top surface, said light generatingsubstructure having an active region and a light-emitting spot, c) aleakage substructure arranged such that a predetermined portion of saidgenerated light leaks out through said top surface; 2) a secondsemiconductor laser diode arranged to emit an output beam having apredetermined output spectrum for use at a predetermined wavelength,said diode including: a) atop surface, b) a light generatingsubstructure including a plurality of semiconductor layers disposedbelow said top surface, said light generating substructure having anactive region and a light-emitting spot, c) a leakage substructurearranged such that a predetermined portion of said generated light leaksout through said top surface; and 3). bonding and supporting structuresfor disposing said first and second laser diodes such that their topsurfaces are facing each other and a predetermined portion of saidleaking light of said first and second laser diodes enters said lightgenerating substructure opposite said leaking light; and B. locking saidgenerated light of said first and second laser diodes in a predeterminedphase relation.
 12. The method according to claim 11 wherein said firstand second laser diodes are disposed such that their output beams havesubstantially parallel irradiation directions.
 13. The method accordingto claim 11 wherein said first and second laser diodes are constructedso that said active region and said light-emitting spot are proximate tosaid top surface.
 14. The method according to claim 11 wherein saidfirst and second laser diodes are disposed such that their top surfacesare within a distance smaller than each of their output wavelengths. 15.The method according to claim 11 wherein at least one of said leakagesubstructures of said first and second laser diodes comprises arelatively thin metal contact layer, said metal contact layer isarranged such that said predetermined portion of said generated lightleaks out through said top surface.
 16. The method according to claim 11wherein at least one of said leakage substructures of said first andsecond laser diodes comprises a plurality of diffractive gratings, saiddiffractive gratings are arranged such that said predetermined portionof said generated light leaks out through said top surface.
 17. Themethod according to claim 11, further including tuning mechanisms fortuning the output wavelength of said first and second laser diodeswithin a predetermined wavelength range, said tuning mechanisms beingarranged to tune at least one of said laser diodes.
 18. The methodaccording to claim 11, further including lens means for coupling saidoutput beams of said first and second laser diodes into a single-modedevice.